There are numerous applications in which it is desirable or necessary to be able to detect when a moveable entity is placed in a predetermined position with respect to another entity, such as a base. There are numerous applications where additionally it is necessary or desirable to be able to identify the moveable entity when it is located in the predetermined position, for example where the moveable entity is one of a number of moveable entities that can each be arranged in the predetermined position.
It is known, for example, to use RF ID tags in such applications. Each tag typically comprises a circuit including an inductor and the base is arranged to generate an electromagnetic field in a defined region. When the RF ID tag is brought into that defined region there is inductive coupling between the base and the RF ID tag. This inductive coupling may be arranged to supply energy to the RF ID tag, which may then transmit a signal to the base, that signal enabling the base to identify the particular RF ID tag.
Whilst such tagging systems find a wide variety of applications, they are relatively complex, and the necessity for each tag to include a respective inductor poses challenges both to attempts to reduce the complexity of the system and to reducing the cost of the tags.